4 Answers
4

There are several aspects of the chemistry of the chocolate cake being converted to a white cake that are not covered in the existing answers.

Fat content of cocoa powder

You cannot simply replace the cocoa powder with flour, because it compared to flour, cocoa powder:

Contains more fat (about 10-12 percent by weight, for grocery store brands; more for some premium brands up to about 25%)

Is more acidic

Is far, far more flavorful (okay, that part is not desired in a white cake)

You should use slightly less flour than there was cocoa, to avoid adding flour by weight equivalent to the fat in the original recipe which would throw the balance off. I would suggest a half cup plus 2 tbls of flour instead of 3/4 cup of cocoa as a starting point.

Flavor balance

With the astringency of the cocoa removed from the recipe, the flavor balance may be too sweet. You might want to try reducing the sugar by about 1/4 cup - 1/2 cup.

Acid and Leavening

The original recipe contains both baking soda (which requires an additional acid from another ingredient) and baking powder as leavening.

When you remove the cocoa, you are removing the acid that the baking soda was intending to react with.

Therefore, you need to remove the baking soda, replacing it with baking powder which brings its own acid. You might need a slightly larger amount, as part of the weight of baking powder is the acid, so there is less leaving per unit than with baking soda. I would suggest an additional 1/2 teaspoon as a starting point.

Boiling water

The boiling water is specifically intended, I infer from experience with many chocolate cake recipes, to bloom the flavor in the cocoa. When you remove the cocoa from the recipe, you remove the need to use boiling water to enhance the chocolate flavor.

Furthermore, the reason water is used rather than milk or another liquid is to interfere less with the perception of chocolate flavor. Therefore, you may choose to use another liquid like milk or additional coconut milk which will enhance the flavor and tenderness of your final cake.

Vegetable Oil

Vegetable oil is chosen in this recipe to avoid interfering with the flavor from the cocoa.

You may continue to use it, or you may find that butter or even coconut oil give a more agreeable flavor for the final product you are creating.

Method

With all of the above changes, there will no doubt be some changes to the method. Since you have only listed the ingredients, and not the actual steps, it is harder to be specific here.

However, what will be left is a fairly standard sponge cake, so should modify the steps to the standard creaming method:

Sift the dry ingredients together; combine the wet ingredients other than butter and sugar

Cream the butter and sugar(s) together

Add in the eggs one at a time, beating until smooth.

Add the dry mix alternating two or three times with the wet mix, beating each time for 30 seconds or so

Beat one or two minutes longer to develop structure

Flavorings

For coconut, using coconut oil in lieu of vegetable, and more coconut milk in lieu of water, and adding coconut extract should do the trick. The recipe should also be able to carry a small amount of shredded coconut if you choose, although that will change the texture.

For lemon, start as with vanilla, and add lemon zest to the batter (and maybe lemon extract), and possibly, should you choose, a touch of yellow food color. I would not add lemon juice as it has a profound effect on the chemistry; instead I would make a nice lemon icing or glaze that has the tartness.

Conclusion

Despite all of the above, I would not try to convert a recipe that is specifically designed for chocolate to a white cake; too many aspects of the cake will change. Instead, I suggest finding a truly outstanding white or yellow cake recipe to use.

This answer needs to be linked to the FAQ for "how to answer". :)
– MartiJun 21 '13 at 19:31

1

it was funny when I stumbled upon this thread since someone had the exact same question as mine on the exact same recipe! I did as suggested here - added 1/2 cup + 2 tbsp flour & followed all the other suggestions. I have also made the cocoa layer so I can compare. The vanilla cake is too bread-y. Too dense, and most definitely too much flour. I wonder if replacing the cocoa powder with some sort of starch is a better idea or just add less flour, for e.g. 1/4 cup more, instead of the above suggestion.
– user47278Jun 9 '16 at 1:48

The usual method for converting a chocolate cake to a plain cake is, as you have done, replace the cocoa with flour. However, if you find this makes the cake too floury, try using perhaps 1/2 or 1/3 cup instead - you may even need to omit the extra flour altogether.

Once you have found the right proportion of extra flour you will have a basic white cake. To make the other variants you would start with this and just add a few things. For example:

Thanks and I'll keep working at it with the suggestions you mentioned.
– YolandaMar 29 '12 at 16:11

Cocoa powder seems to have somewhat below 60% carbohydrates. So the correct replacement should be around 60g cornstarch. You can also use 65 to 70 g flour, but it will make the cake a bit chewier.
– rumtscho♦Mar 29 '12 at 17:30

So I am using cornstarch instead of flour?
– YolandaMar 29 '12 at 18:19

@yolanda both will work, I suppose that cornstarch will be slightly better, but I don't know if the difference will be enough to notice.
– rumtscho♦Mar 29 '12 at 23:35

I agree with some points that ElendilTheTall has highlighted but I would like to add several different tips on converting the chocolate cake to a different flavored cake.

First, instead of substituting the cocao powder with more flour, I would instead substitute a packet of instant pudding mix that matches the flavor of the cake you are trying to convert to. There are definitely vanilla and lemon flavored pudding mix but not sure about coconut. They add great flavor and moisture to your cake. If there isn't the flavor of pudding that matches your cake, than it is safe to replace with just more flour.

Second, I would probably take out coconut milk and replace it with cow milk except for the coconut flavored cake. Chocolate has a strong flavor and can withstand being paired with coconut milk. But I am not sure a vanilla cake can be considered vanilla cake if there is actually coconut milk in it.

If you follow the above tips as a base and then add additional flavor extracts or zest to your cake, you can effectively convert your chocolate cake into whatever flavor cake you want.